The early detection of invading viruses by the host depends on a limited number of specific intracellular receptors that detect viral patterns and activate signal transduction cascades, thereby triggering interferon (IFN)-mediated anti-viral defense mechanisms. Key virus-detecting receptors include the nucleic acid recognizing Toll-like receptors and the cytosolic RNA receptors RIG-I and MDA-5. Specifically, RIG-I has emerged as a key receptor in sensing emerging viruses, including the influenza virus and hepatitis virus C (HCV), whereas MDA5 responds to the infection of picornaviruses and noroviruses. Members of the tripartite motif (TRIM) protein family also play major roles in the inhibition of the lifecycles of viruses. On the other hand, the efficacy of the IFN responses has led to a number of viruses evolving various evasion strategies against the host IFN system. HCV and influenza A virus are emerging pathogens associated with severe liver disease and fatal respiratory diseases, respectively. Immune evasion strategies of influenza virus and HCV are essential to escape the host anti-viral response, allowing chronic or sustained viral replication in vivo and resulting in severe and sometimes deadly diseases. This multi-project Cooperative Agreement (U19) grant application will attempt to address how the host develops firsthand innate immune recognition, how it is activated in response to viral infections, and how viruses have evolved the multiple mechanisms necessary to thwart the host innate defense. This application consists of five projects with multidisciplinary schemes. Project 1 utilizes cell biological and immunological approaches to study RIG-I and TRIM25 mediated immune surveillance against virus; Project 2 focuses on the structural basis of RIG-I and MDA-5 mediated anti-viral response; Project 3 uses single molecule fluorescence techniques in vitro and super-resolution imaging of cells to dissect the RIG-I pathway; Project 4 investigates how HCV interacts with host cells to evade intracellular innate immunity; and Project 5 studies how the influenza A virus NS1 protein prevents cytoplasmic viral sensors from recognizing viral infections, us, the goal of this U19 project is to unravel the key molecular events of host immune recognition and viral immune escape, which ultimately give insights for novel antiviral therapeutic interventions and safe and effective vaccine development. RELEVANCE (See instructions): Investigation of the cross-talk between the cytosolic viral receptor pathways to induce the IFN-mediated antiviral action and the evasion strategies of emerging viruses to blunt host innate immunity provides the foundation for the development of novel diagnostic, therapeutic, and vaccine approaches for emerging virus-associated disorders. PROJECT 1: RIG-I and TRIM 25 mediated immune surveillance against viruses (Jung, J) PROJECT 1 DESCRIPTION (provided by applicant): Understanding host-viral interaction is an essential step in developing safe and effective antimicrobials against biodefense agents and emerging pathogens. Early detection of invading viruses by the host depends on a limited number of specific intracellular receptors that detect viral patterns and activate signal transduction cascades, thereby triggering interferon (IFN)-mediated anti-viral defense mechanisms. Retinoic acid-inducible gene I (RIG-I) has emerged as a key cytosolic viral RNA receptor for sensing emerging viruses, including the influenza virus and hepatitis virus C (HCV). In addition, members of the tripartite motif (TRIM) protein family, containing a RING-finger domain, B box/coiled-coil domain (B Box/CCD), and a SPRY domain, play a major role in the inhibition of the lifecycles of viruses. Furthermore, the interconnection between the RIG-I and TRIM family is required to initiate the induction of the protective IFN-mediated host anti-viral innate immunity. Our collaborative works have demonstrated that the RIG-I-mediated IFN pathway requires multiple step processes: upon viral infection, the C-terminal regulatory domain (RD) of RIG-I recognizes viral RNA in a 5'-triphosphate-dependent manner, leading to RIG-I dimerization and ATPase activity. Subsequently, RIG-I undergo a robust ubiquitination induced by the TRIM25 E3 ligase, enabling RIG-I to interact with the downstream CARD-containing mitochondrial anti-viral signaling (MAVS) protein and thereby inducing antiviral signal transduction to limit viral replication and transmission. The goal of this study focuses on better understanding the regulation of RIG-I and TRIM25 pathways: how posttranslational modifications affect RIG-I and TRIM25 signaling activity (Aim 1), what the modes of feedback regulation for the RIG-I and TRIM25 pathways are (Aim 2), and finally, what roles RIG-I and TRIM25 mediated immune surveillance have against viruses (Aim 3). Thus, the proposed study will attempt to delineate the molecular mechanisms underlying the host-viral interaction at a basic scientific level and will also provide the foundations for developing novel diagnostic and therapeutic strategies for emerging virus-associated disorders at the public health level.